Wrought zinc product



Patented Dec. 8, 1931 UNITED STATES PATENT OFFICE EDMUND A, ANDERSON ANDELIHU H. KELTON, 0F PALMER-TON, PENNSYLVANIA, A8-

SIGN OBS TO THE NEW JERSEY ZINC COMPANY, OF NEW YORK, N. Y., ACORPORATION or NEW Jsasnx No Drawing.

This invention relates to wrought or mechanically worked zinc products,such for example as rolled sheet or strip zinc, and has for its objectthe provision of a method of improving the physical properties ofwrought zinc products madefrom a zinc base capable of being worked intowrought allqy pro ucts possessing superior resistance to cold' flow.More particularly, the invention aims to provide a method of improvingthe dynamic ductility of such wrought products in order to permitsuccessful conduct of certain subsequent fabricating or formingoperations, such as punch-pressing, folding, bending etc. Again, theinvention aims to provide a method of improving the resistance to coldflow of such Wrought zinc products where, as a consequence of apreceding mechanical working operation, the wrought product does notpossess such superior resistance to cold flow as it might. The method ofthe invention may be practiced primarily for improving either or both ofthese particular physical properties of the wrought zinc product.

Zinc, like other metals of relatively low -melting point, undergoes slowplastlc deformation or cold flow when subjected continuously to loads aslow as a quarter of the ultimate tensile strength as measured by or-vdinary tensile testing methods. All Wrought or mechanically worked zincproducts made of high grade or common zinc metal are readily susceptibleto such plastic or progressive and permanent deformation under constantand continuously applied loads materially below the ultimate tensilestrength,--a phenomenon frequently designated as cold flow. Inotherwords, at ordinary temperatures a continuously applied constant load(far below the ultimate tensile strength as determined by ordinarymethods) causes permanent deformation in the heretofore availablewrought zinc products of commerce. Under sufficiently low continuousloads the rate of progressive deformation becomes so small as to beunmeasurable by known methods, if not actually reaching the zero value.and such low or negligible loads may be interpreted as safe workingstresses for these heretofore WROUGIIT ZINC PRODUCT Application filedMarch 14, 1928. Serial No. 847,196.

available wrought zinc products when used as structural materials, forexample, in the form of corrugated sheets. From the structural engineersviewpoint, however, these wrought zinc products have so low a safeworking stress, as determined by actual practical experience, as toseriously restrict their commercial application.

Certain zinc base alloys have recently been discovered to possess thecapacity of being wrought or mechanically worked into zinc productspossessing superior resistance to cold flow, as compared with wroughtzinc products made from either high grade or common zinc metal.Theprinciples involved in the compounding of such zinc base alloys aredisclosed in the copending patent application of Willis M. Peirce andEdmund A. Anderson, Serial No. 346,493; filed March 12, 1929. Briefly,such zinc base alloys contain one (or more) metallic element which goesinto solid solution in the zinc to a measurable extent and one (or more)other metallic element which is present in the alloy in an amountgreater than its limit of solid solubility in binary association withzinc at ordinary room temperature, say 20 C'. A zinc base alloy is to behere understood as one consisting principally of zinc, say for examplenot less than zinc and preferably not less than zinc. The element (orelements) in solid solution in the zinc may be copper, cadmium,manganese, aluminum, and probably others. Cf these the most satisfactoryresults have been secured with copper, cadmium and manganese. Theelement (or elements) present inexcess of its limit of solid solubilityin inc may be magnesium, lithium, manganese, nickel, and probablyothers.

The following list of alloys is here included merely as examples of suchzinc base alloys capable when appropriately wroughtwhile the balance ofeach of the other alloys was high grade zinc metal.

The percentages of the alloying elements present in the zinc base alloyare susceptible of variation over a certain range, as discussed in theaforementioned patent application. From thatdiscussion, it appears that,with appropriate methods of mechanical working, the alloying elementsmentioned in the foregoing list may vary within the following limits:copper or cadmium from 0.05 to 2% and possibly to 5%; manganese (asfirst element within limit of solid solubility in zinc) from 0.01 to0.1%, and (as second element exceeding limit of solid solubility inzinc) from 0.1 to 2%; magnesium or lithium from 0.005 to 0.5% and nickelfrom 0.05 to 1-%.

While the zinc base alloys hereinbefore described can be wrought ormechanically worked bythe methods or practices heretofore customarilyused in mechanically working high grade or common zinc metals, it hasbeen found that these methods and practices do not ordinarily developthe optimum resistance to cold flow which these zinc base alloys arecapable of imparting to wrought zinc products. Thus, Willis M. Peircehas disclosed in his copending patent applica tion, Serial No. 347,195,filed March 14,1929, a method of developing superior resistance to coldflow in,wrought zinc products made from such zinc base alloys by hotmechanical working of the'alloy with the view of substantiallyinhibiting any condition of work hardening in the finished wroughtproduct. While'it appears to be the fact that the resistance to coldflow of wrought products made from these zinc base alloys isdeleteriously affected when any condition. of work hardening existstherein, such work hardening can be subsequently removed by appropriateheat treatment of the wrought prodnot, and we have found that such heattreatment produces an attendant improvement in the resistance to coldflow of the {so heattreated wrought product.

Commercial wrought zinc products, are often produced by two or moredifferent mechanical working operations, The first or primary workingoperation .is designed to produce suitable'forms or blanks of the metalfor the subsequent secondary or fabricatin operations. Thus, in themanufacture 0 corrugated sheet zinc roofing, the metal is first rolledinto sheetsthe primary working operation, and is then bent orcorrugatedthe secondary or fabricating operation.

It has been found in practice that even when the zinc base alloyshereinbefore described have been appropriately wrought to develop theoptimum resistance to cold flow, the wrought product may be deficient incertain other physical properties essential to successful subsequentfabrication or forming. In a general way, the ability of the wroughtzinc product to successfully undergo various fabricating and formingoperations is indicated by its dynamic ductility,the higher the dynamicductility the better suited being the product for successfullywithstanding such operations as bending, folding, punchpressing and thelike. We have found that deficient or inferior dynamic ductility inwrought zinc products (of the character herein contemplated) may beameliorated by the heat treatment hereinbefore mentioned and now to bedescribed in detail.

The present invention accordingly involves a novel heat treatment ofwrought zinc products made of such zinc base alloys as hereinbeforedescribed. This heat treatment may be advantageously applied to suchwrought products to remove work hardening resulting from a precedingworking treatment, for the purpose of improving the resistance to coldflow of the resulting heat-t reated wrought product. Again, the heattreatment of the invention may be advantageously applied to any wroughtproduct of these zinc base alloys, irrespective of its condition of workhardening, for the purpose of improving certain physical properties,such as dynamic ductility, essential to the successful conduct ofvarious subsequent fabricating or forming operations, such aspunch-pressing or forming operations like drawing, extruding, squirting,spinning, bending, folding, etc. Obviously, the heat treatment may atoneand the same time'remove a condition of work hardening and improve otherphysical properties, such as dynamic ductility.

The heat treatment of the invention is characterized by rapidly heatingthe wrought zinc product (madeof a zinc base alloy of the characterhereinbefore described) to the predetermined Y elevated, temperature ofheat treatment, 'arid holding the product at that temperature forasufficient length of time to produce theidesired contemplated change inits physical properties. i The essential features of this heat treatmentare (1) that the wrought product be brought to and maintained at therequired heat treating temperature for such time "as'is necessarytoremove work hardening or toefiect-o'ther advantageous changes in itsphvsical properties,

and (2) that the heating of the wrough product from its initial (usuallyroom) tempera ture to the minimum temperature of the permissable rangeof heat treatment temperature be brought about very rapidly, preferablyin 20 seconds or less. Following this heat treatment the wrought productmay be cooled in any appropriate manner.

In the rapid heating of the wrought zinc product of the heat treatmenttemperature, it is important that the entire mass of the wrought productbe brought to that tempera ture in as short an interval of time aspossible. Accordingly, the wrought product should be exposed to theheatlng medium in a form capable of promptly and rapidly absorbing heat,so that the necessary rapid heating may take place promptly anduniformly throughout the entire mass or bulk of the product. In ourpreferred practice, We aim to bring the wrought product to the heattreatment temperature in 20 seconds or less. While somewhat longer timeintervals may, in some instances, be tolerated, we have found that themore rapidly the product is brought to the heat treatment temperaturethe better will be the results.

The minimum or critical temperature to which the wrought product shouldbe brought for the purposes of the invention cannot be precisely statedsince it varies not onlv with the particular composition of the alloybut with the previous working treatment. Our experience indicates thattemperatures from 200 C. to 400 C. are ordinarily satisfactory for thepurpose. In a general way, the higher the temperature the shorter is thetime required to bring about the desired change in the physicalproperties of the wrought product. Havin rapidly brought the wroughtproduct to t e correct elevated temperature of heat treatment, thecontemplated change in its physical properties proceeds with alacrity.With temperatures in the neighborhood of 300 (3., the desired change inphysical properties takes place in one minute or perhaps less, while attemperatures of about 200 C, 5 to 10 minutes may be required. Foreconomic reasons, we generally prefer the higher temperatures andshorter times of heat treatment.

Prolonged maintenance of the wrought product at the high temperature ofheat treatment produces no harmful aifect on the resistance to coldflow, dynamic ductility, or equivalent physical properties, of the heattreated wrought product. Indeed, after the contemplated change inphysical properties of the wrought product has been brought about by theheat treatment of the invention, continued exposure of the product tothe elevated temperature of heat treatment appears to be without furthereffect on the product. Consequently, while prolonged heating does noharm, the aim in practice is to continue the heat treatment only so longas is necessary to bring about the contemplated change in physicalproperties of the wrought product.

' The cooling of the heat treated wrought product may be effected in anyappropriate manner; So far as concerns those physical properties withwhich we are here interested, the manner and character of cooling isimmaterial. Accordingly, after the completion of the heat treatment, thewrought product may;i be cooled rapidly or gradually as desir The heattreatment may be carried out in any appropriate apparatus designed torapidlyheat the entire mass of the wrought product to the requiredelevated temperature of heat treatment. Thus, the heat treatment mayadvantageously be conducted by immersing the wrought product in a bathof oil or other suitable liquid heated to the required heat treatmenttemperature. Various types and forms of annealing furnaces designed forrapid heating of materials may be used. The heat treatment may becarried out as an intermittent or continuous operation, depending u onthe manner in which the wrought pro uct is brought into contact with orexposed to the heating medium.

Where the .invention is practiced primarily for the purpose of removingwork hardening, it is generally desirable that the condition of workhardening in the wrought product be substantial in order to providenumerous nuclei for readjustment of the crystalline structure in thesubsequent heat treatment. While the heat treatment of the invention isapplicable for removing any condition, however slight, of work hardeningin the Wrought product, the readjustment of the crystalline structurebrought about as a consequence of the heat treatment is promoted by auniform distribution of numerous nuclei about which such readjustmentmay originate and pro ress. It is for this reason that we find itdesirable, where practicin the invention primarily for removing workardening, to deliberately impart to the wrought product a substantialening in the course of the treatment.

Work hardening results from the mechanical working of a metal atrelatively low temperatures. The temperatures at which metals should beworked to produce work hardening in the wrought product difierconsiderably with different metals. In general, it may be said that workhardening resultsfrom the mechanical working of the metal attemperatures below the recrystallization temperature of the metal, whichmay be defined as the temperature at which the metal rapidly returns toan unstraincd crystalline structure.

The recrystallization temperature of the zinc base alloys hereinbeforedescribed appearstobeintheneighborhood of 175200 C.

preceding working condition of work hard- Consequently, mechanicalworking of these alloys at temperatures well below 175 C. produces inthe resulting wrought product a substantial condition of work hardening.The degree or amount of work hardening in the wrought product willdepend upon the composition of the alloy as well as upon the character,particularly the temperature, of the working treatment.

The invention is particularly adapted for the development of superiorresistance to cold flow in cold worked or wrought products. As anexample, of such an application of the invention, we will described therollingof strip or sheet zinc. The cast slab, of a zinc base alloy suchas herein contemplated, is first subjected to the usual homogenizinganneal, involving the exposure of the slab to a temperature of about 200C. in an appropriate annealing furnace for from 8 to 24 hours. The slabis then transferred to the rolling mill, and the rolling treatment isconducted to produce a substantial condition of work hardening in thefinished strip or sheet. Any cold rolling practice will bring aboutthis. result. The metal may be rolled in the form of continuous stripsor sheets or in the form of packs as will be well understood in the art.

The rolled product is now subject to the heat treatment characteristicof the inven- To this end, the strips or sheets are preferably heated toabout 300 C. in about 20 seconds or less. In order to obtain this rapidheating of the strips or sheets, it is desirable to expose them to theheating medium in the form in which they will most readily absorb heat.For this reason, it is preferable not to coil or stack the strips orsheets, but rather to expose as much surface as practicable tothe'heatingmedium at the instant of subjecting the strips or sheetsthereto.

In our investigations, we have used the static tensile strength of thewrought zinc product as a measure of its resistance to cold flow. Thestatic tensile strength may be conveniently measured by applying astatic or dead load to a suitable test specimen and observing the rateof elongation at intervals over an extended period of time; A series ofsuch tests made with loads giving varying tion.

stresses in pounds per square inch is requiredfor the completeevaluation of the static tensile strength. A similar method formeasuring the analogous phenomenon of creep in steel is described byFrench in Technological papers of the Bureau of Standards No. 296.

The following table shows the results of static tensile tests upon twowrought zinc products made of the same zinc base alloy. The alloy wascomposed of 1% copper, 0.01% magnesium, and the balance common zincmetal. Specimen A was cold rolled; that is with a Static tensilestrength expressed as Per cent elongation,

with load of 15,000

lb i .i with load oi 5 per Sq n R 10,000 lbs. per sq. in.

3,000 min. 9,000 min.

Specimen A 15, 000 2. 75 9.0 Specimen B over 00, 000 0.75

The numerical figures of static tensile strength given in the foregoingtable are expressed, iirst, as the time in minutes to produce 10%elongation in a standard test specimen at a temperature of 20-25 C. witha dead load calculated to give a stress of 10,000 pounds per square inchon the original section of the test specimen, and, second, as thepercent elongation of the test specimen in (a) 3,000 minutes and in (b)9,000 minutes dead load calculated to give a stress of 15,000 pounds persquare inch. The standard test specimen was a representative section ofthe wrought zinc products (rolled sheet zinc in these instances), 0.032inch thick, 2 inch gauge length, inch reduced section width, 1 inch widegrips, and 1 inch radius fillets.

It is commonly known that certain physical properties of wrought zincproducts, made of the ordinary commercial grades of zinc metal, areusually deleteriously afiected by annealing. For example, annealing overa wide range of temperatures (150300 C; is usually deleterious to thedynamic ducti ity of rolled strip or sheet zinc made of either highgrade or common zinc metal. In the light of these known facts, it wouldnot be expected that heat treatment could improve the dynamic ductilityof wrought zinc products made from the zinc base alloys with which weare here concerned. However, we have abundantly demonstrated that theheat treatment of the invention does accomplish such an improvement inwrought products made from such zinc base alloys.

The following table indicates the improvement brought about by the heattreatment of the invention in the dynamic ductility of two specimens ofwrought zinc made from a zinc base alloy composed of 1% copper, 0.01%magnesium and the balance common zinc metal. (For test method see A. S.T. M. Tentative Standards 1928, page 158). Specimens A were rolled tolow hardness and cold flow as possible.

high ductility-essentially hot rolling; while specimens B were rolled tohigh hardness and low ductility essentially cold rolling. In each caseof heat-treatment, the wrought product was raised from room temperatureto the annealing temperature in less than 20 seconds:

While the dynamic ductility has been used as an example of a mechanicalproperty of the wrought zinc product which may be beneficially affectedby the heat treatment of. the invention, other physical properties suchas cold bends, percent elongation, etc., may likewise be improved.

lhe heat treatment of the invention may be advantageously applied atvarious stages in the course of the complete mechanical working of thewrought zinc product. Thus, it may be. applied to rolled sheet or stripzinc to remove work hardenin and hence improve the resistance to cold owor to increase the dynamic ductility or for both purposes. Also, it maybe applied between different fabricating or forming operations toovercome a decrease in dynamic ductility oc f casioned by a precedingoperation, in order to permit the successful conduct of a furtherfabricating operation or operations. Again, it may be applied to thefinished wrought zinc product to remove any condition of work hardeningcaused by any preceding mechanical working, with the view of impartingto the finished product as great a resistance to We claim 1. The methodof improving the physical properties of a wrought zinc product made om azinc base alloy capable of being worked into a wrought productpossessing marked resistance to cold flow, which comprises subjectingthe wrought product to heat treatment characterized by rapidly heatingthe wrought product to an elevated temperature of about 300 (3., andmaintaining the wrought product at that temperature until thecontemplated improvement in its physical properties has taken place,said temperature being adapted to avoid work hardening 1n the wroughtproduct while improving its 00 resistance to slow plastic deformation orcold flow and increasing its dynamic ductility.

2. The method of improving the physical pxropertiesof a wrought zincroduct made om a zinc base aloy capa le of being 85 worked into awrought product possessing worked into a-wrought 3. The method ofimproving the physical properties of a wrought zinc product-made from azinc base alloy capable of being product possessing marked resistance tocold flow, which comprises subjecting the wrought zinc product to heattreatment characterized by heating the wrou ht product to an elevatedtemperature of a ut 300 C. in a time interval not exceeding 20 secondsand maintaining the wrought product at that temperature until thecontemplated improvement in its physical properties has taken place.

a. The method of improving the physical properties of a wrought zincproduct made from a zinc base alloy capable of being worked into awrought product possessing marked resistance to cold flow, whichcomprises heating the wrought product to a temperature of 200400 in atime interval not exceeding 20 seconds and maintalning the wroughtproduct at that temperature until the contemplated improvement in itsphysical properties has taken place.

5. The method of improving the physical properties of a wrought zincproduct made rom a zinc base alloy capable of being worked into awrought roduct possessing marked resistance to cold flow, whichcomprises heating the WrouCght product to a temperature of 200-400 in atime mteryal not exceeding 20 seconds and maintaining the wroughtproduct at that temperature for a period of 1 to 10 minutes.

6. The method of developing superior resistance to cold flow in awrought zinc prodnot made from a zinc base alloy of suitable compositionfor the purpose, which comprises subjecting the wrought product to heattreatment characterized by rapidly heating the wrought product to anelevated temperature of 200300 0., and maintaining the wrought productat that temperature until the contemplated superior resistance to coldflow has been developed.

In testimony whereof we aflix our signatures.

EDMUND A. ANDERSON. ELIHU H. KELTON.

